Question:
When using river transportation capability, not RT units, do you have to
plan an invasion 1 turn in advance if intending to land in
enemy-controlled hexes?

Answer:
No, as river movement into enemy territory is
not an amphibious landing. Design Note: River movement uses local small
craft that can be rounded up in a very short period of time, without
giving the enemy any real warning; using a RT or regular naval units to
make an amphibious landing is a much more involved process.

Source:
Arthur E. Goodwin posted on the Europa Association mailing List at Yahoo,
11:47am Jun 27, 2002 PST